1. Field of the Invention
The present invention relates to a dual digital television tuner having two systems.
2. Description of the Related Art
FIG. 4 shows the configuration of a known dual digital television tuner. A digital television intermediate-frequency signal (IF), which is generated by frequency-conversion by an outside downconverter (not shown), is separated and distributed by a distributor 51, and one separated intermediate-frequency signal is input to a first direct conversion circuit 52. The first direct conversion circuit 52 includes a first intermediate-frequency amplifier 53 to which the one separated intermediate-frequency signal is input, a first variable gain amplifier 54 provided in the next stage, a pair of first mixers 55 and 56 provided in the next stage, a first local oscillator 57 for generating a local oscillation signal, a first phase shifter 58 for dividing the local oscillation signal into two phases orthogonal to each other and for inputting the phases to the mixers 55 and 56, and a pair of first baseband signal amplifiers 59 and 60 provided in the stage after the mixers 55 and 56.
Outside the first direct conversion circuit 52, a first PLL circuit 61 for controlling the oscillation frequency of the first local oscillator 57, a first demodulator 62 for demodulating baseband signals (I/Q), and so forth are provided.
On the other hand, the other separated intermediate-frequency signal output from the distributor 51 is input to a second direct conversion circuit 63. The second direct conversion circuit 63 includes a second intermediate-frequency amplifier 64 to which the other intermediate-frequency signal is input, a second variable gain amplifier 65 provided in the next stage, a pair of second mixers 66 and 67 provided in the next stage, a second local oscillator 68 for generating a local oscillation signal, a second phase shifter 69 for dividing the local oscillation signal into two phases orthogonal to each other and for inputting the phases to the mixers 66 and 67, and a pair of second baseband signal amplifiers 70 and 71 provided in the stage after the mixers 66 and 67.
Outside the second direct conversion circuit 63, a second PLL circuit 72 for controlling the oscillation frequency of the second local oscillator 68, a second demodulator 73 for demodulating baseband signals (I/Q), and so forth are provided.
The local oscillators 57 and 68 are controlled so that they oscillate at substantially the same frequency as the intermediate frequency of the channel that each of the direct conversion circuits 52 and 63 should receive. In the first direct conversion circuit 52, the pair of mixers 55 and 56 receive local oscillation signals whose phases are orthogonal to each other, and thus the baseband signal amplifiers 59 and 60 output baseband signals which are orthogonal to each other.
Likewise, in the second direct conversion circuit 63, the pair of mixers 66 and 67 receive local oscillation signals whose phases are orthogonal to each other, and thus the baseband signal amplifiers 70 and 71 output baseband signals which are orthogonal to each other.
The baseband signals are added and demodulated in the demodulators 62 and 73, respectively, converted to an analog signal, and converted to a picture signal or the like.
In the above-described configuration, when the first direct conversion circuit 52 and the second direct conversion circuit 63 receive intermediate-frequency signals of the same channel, the oscillation frequency of the first local oscillator 57 is the same as that of the second local oscillator 68. Thus, mutual interference between the local oscillation signals causes beating (which is due to a slight difference that exists between the local oscillation signals), leading to a deterioration of the phase noise of the local oscillation signals and degradation of the reception characteristics.